A worldwide concern for increasing global levels of carbon dioxide (CO2) in the atmosphere has emerged in the last ten years. Countries and governments are continuously trying to implement regulatory frameworks in order to incite efforts for reducing the overall emissions of CO2 and/or equivalent greenhouse gases. Biological carbon sequestration though photosynthesis is a natural way to recycle carbon that has been recently extensively explored for addressing this problem.
Further, worldwide fossil fuel deposit depletion has pushed researching for alternatives to products that are currently processed from petroleum. In certain applications where high amounts of fuels are needed remotely from sources of supply (e.g. forward military bases or remote exploratory camps experience), costs associated with conventional fuels are high, primarily due to expenses involved in fuel delivery and associated pollution due to transportation means. Therefore, alternatives to produce fuels at the point of use, rather than transporting them to the desired site have been investigated to reduce those costs. In this effort, biofuels such as biodiesel have been identified as a possible alternative for replacing fossil fuel consumption without increasing the CO2 content of the atmosphere. However, the process involved in creating biofuel from biomass is expensive relative to the process of extracting and refining petroleum.
A number of strategies are focused on methods to increase carbon dioxide uptake in biological systems such as green plants through sunlight and CO2 uptake while research went on for optimizing production yields, diversifying and valorising the biomass by-products resulting from photosynthesis. However, the industrial development of those strategies has been hampered by many difficulties in transposing those experimental methods into scalable and/or cost effective solutions. In particular, control of the main parameters affecting the rate of photosynthesis, e.g. a favorable temperature, intensity and wavelength of light, and availability of nutrients such as carbon dioxide has proven to be delicate for closed system applications (e.g. photobioreactors), whereas open ponds to grow biomass suffer from risks of contamination and exhibit high operating costs.
Among phototrophic microorganisms, microalgae is one of the most efficient organisms for converting solar energy using carbon dioxide as growth nutrient and is an efficient producer of oxygen and biomass. Valuable components such as carbohydrates, sugars, proteins and fat can be harvested from the biomass and directly or indirectly converted into high value added products such as pharmaceutical products, nutraceutials, cosmetic products, food products, fine chemicals usually synthesized in chemical plants, or energy supplies such as methane or more interestingly biodiesel and other fuels used in turbines and/or thermal cycle engines for generating movement, in transportation, essentially.
It is known that microalgae productivity is limited by three major factors: availability of light and nutrients, and temperature. Historically, most efforts have been invested in developing the optimum nutrients for any specific microalgae, notably by saturating the photosynthesizing system with CO2. Land-based (e.g. ponds) microalgae culture plants, while showing some effectiveness in capturing CO2, are limited by available land space, water supplies (mainly due to evaporation), external contamination (e.g. other species, bird dejections), productivity (not operable at night) and costs associated with the processing of huge quantities of water. Optimal temperature conditions for efficient biomass production are usually selected in accordance with the climatic conditions prevailing in a chosen site. Yet, even in such sites, winter and night temperatures, as well as morning hour temperatures pose serious limitations to growth rates.
Further, UV exposure of the microalgal culture in outdoors production plants results in the oxidation of the microalgae at the surface of the water. Attempts to solve these problems led to the creation of shallow ponds or raceways. However, such shallow water approaches engender high evaporation and saline deposits, which also reduce the efficacy of continuous outdoor growth. Overall, weather, diurnal cycles, invasion by opportunistic species and external pollutions further aggravate the difficulties of mass microalgae culturing in outdoor settings.
Photobioreactors (PBRs) for photosynthesizing biomass culture provide a compact infrastructure designed to address the above problems. The scale-up of photobioreactors to achieve a commercially viable production of algae products is hampered by the limitation of available lighting, both in terms of light delivery and distribution and of energy expenditure. For instance, current methods of mass cultivation of marine algae include translucent fiberglass cylinders, polyethylene bags, carboys and tanks under artificial lighting or natural illumination in greenhouses. During the microalgae growing process the organisms multiply and the culture density increases, and light ends up not being able to penetrate below a few centimeters of depth below the surface of the algae culture thereby decreasing the volumetric productivity of the system.
U.S. Pat. No. 3,520,081 discloses a rotating tank that enhances contact between microalgae and light to accelerate microalgae growth. While such rotating tanks have some benefits, the impracticality of scale becomes apparent when addressing very large scale systems, e.g., multi million gallon systems. U.S. Pat. No. 6,579,714 describes a microalgae culture apparatus and method utilizing a growth apparatus having spaced apart inner and outer walls which are dome-shaped, conical, or cylindrical. Light can pass through the walls into the space between where the algae are cultured. U.S. Pat. No. 5,958,761 describes a tubular bioreactor including a tubular housing surrounding a tubular envelope made in a translucent material and defining a space there between to be filled with a fluid of selective refractive index and the radiation concentration power into the reactor is controlled by modifying the refractive index of the fluid. U.S. 2009/0029445 discloses a biological growth reactor comprising a mixer, a mixing chamber and a reaction chamber comprising a light distributing and fluid dispensing rod. U.S. 2009/0291485 discloses a culture system comprising a culture tank, a rotatable light array and a rotational drive.
However, in current PBRs, the costs in lighting and energy requirements have made prior solutions impractical for all but the culturing of organisms used in high value products such as pharmaceuticals, cosmetics, food products and/or neutraceuticals. Therefore, in spite of higher yields in microalgae culture obtained in some tightly controlled laboratory experiments, heretofore, efforts at microalgal mass culture have been disappointing in that they were inefficient and uneconomical and in particular current microalgae processing methods have failed to result in cost effective microalgae-derived biofuels.